Random freezings in Pi 3b with external usb disk


i have moved my user data folder to an external usb drive (2.5’’ WD 320 gb with external case) and the Pi freezes randomly every day.
after the last freeze, i rebooted the Pi (at 19:28) and i ran the journalctl -r and the dmesg commands.
the only red lines in dmesg are shown in the attached picture.

Could anyone identify any possible root causes?
The Pi PSU is 5V/2.5A (not the official one).